http://belfry.scicalc.com
Very nicely done (so now I won't have to write it myself
Supports both algebraic and RPN modes, and has numerous functions. A lot of nice details, such as shadowed text to give it a truly 3-D feel.
Also supports programmer's mode (binary, hex, and octal), etc.
My only beef is that in RPN mode the stack seems to clear sometimes when I don't expect it to. (for example: 4 enter 4 enter + +. I expect the answer to stay on the stack now when I enter new numbers. simple to solve with an enter, of course).